A court of the United States may not grant an injunction to stay proceedings in a state court except as expressly authorized by Act of Congress, or where necessary in aid of its jurisdiction, or to protect or effectuate its judgments.[1]

A body of case law has grown with respect to the questions of what constitutes an express authorization; what is necessary in aid of jurisdiction; and what constitutes protection or effectuation of a judgment.[citation needed] Cases have also hinged on questions of what orders constitute an injunction by the federal court against the state court.[citation needed]

Among the statutes recognized as express authorization to grant an injunction under the first exception[2]:2 is Section 1983 of the Civil Rights Act.[3] The second exception is used in two areas: to prevent state courts from continuing to hear matters which have been removed to federal court, as well as to permit federal courts to enjoin state suits in cases of real property dispute where a federal claim is filed first. It is relatively rare in application. The third exception is used to prevent parties who have lost in federal litigation from seeking hearing of their cases in state court.

The Supreme Court case Younger v. Harris, while addressing questions distinct from the Anti-Injunction Act, is relevant to the subject of federal injunctions against state court proceedings, insofar as it (and its progeny) enunciates additional doctrines relating to federal courts' injunctive power.[citation needed]